1. Field of the Invention
This invention relates to web transport systems and more particularly to a professional audio magnetic tape transport capstan control system.
2. Description of the Prior Art
Web transport control systems are utilized to control web member motion in a large number of systems ranging from heavy industrial applications such as steel rolling mills to motion picture film transports. Because of the more demanding requirements for extremely rapid acceleration, combined with precise velocity control, magnetic tape transports have particularly been the subject of a great deal of development work. Typically in these systems tape extends along a low inertia tape path from a supply reel through a supply buffer to recording transducers and a capstan and then through a take-up buffer to a take-up reel. The capstan engages the tape to provide bidirectional control of tape motion past the recording transducers in some or all modes of operation. In instrumentation recorders precision steady state speed control is emphasized while rapid and precise acceleration control is emphasized in digital tape transports.
Although use of larger capstans is not totally unknown, professional audio tape transports typically utilize a small diameter capstan such as the capstan motor shaft itself with a pinch roller insuring nonslip engagement with the tape. The small capstan, combined with substantial inertia permit good control over steady state recording velocities. However, for high speed shuttle operation the tape must be released from frictional engagement with the capstan and otherwise controlled. Acceleration to recording velocities typically takes about 3 seconds and consumes a considerable length of tape. While rapid acceleration has heretofore been considered less important in a trade-off against more precise speed control it is nonetheless an extreme inconvenience in some circumstances.
For example, if a radio station wishes to play a recorded message at a given time, the operator must insure that several inches of blank tape precede the recorded message, start the recorder several seconds before the actual message start time, and preposition the tape so that the recorded message reaches the read head at the given time but several seconds after the recorder is started. A miscalculation of timing or positioning can result in the beginning of the message being played too early or too late and it may even be garbled if the recorder has not established the proper tape speed as the message reaches the read head. Similar problems may of course be encountered in trying to record a message at a precise location on a tape.